The objective of the proposed research is to develop novel, selective, and potent ligands for the opiatereceptor-like receptor (ORL1). ORL1 has a primary structure closely related to the opiate receptors, mu, delta and kappa. Although it is clearly in the opiate receptor family, opioid ligands do not bind to this receptor, nor does receptor activation mediate analgesia in the same manner as the other family members. The physiological functions of ORL1 and its endogenous ligand, nociceptin/orphanin FQ (N/OFQ), are not well understood, but it is clear that receptor activation can modulate pain as well as other CNS-mediated responses. In addition, N/OFQ's ability to modulate dopamine release as well as morphine conditioned place preference suggests that a role for novel ORL1 ligands as both pain and addiction medications. This application describes a medicinal chemistry approach to the design and identification of small-molecule, high affinity, selective ORL1 agonists and antagonists, based on a novel potent lead compound designed in our laboratories using computer-assisted drug design and synthesis. Our approach provides a basis for structure activity relationship (SAR) and computer-assisted modeling studies designed to develop new ligands for ORL1 based on novel templates. The compounds to be synthesized will contain sufficient structural variability to sample the ORL1 receptor binding pocket and thus define a pharmacophore for ORL1 and to better understand the differences between opioid receptor and ORL1 binding. All compounds synthesized will be tested for binding affinity at ORL1 and opioid receptors on human receptors expressed in CHO cells. Functional activity will be determined in two in vitro assays: stimulation of [35S]GTPgammaS binding to CHO cell membranes, and inhibition of cAMP accumulation in intact cells. High affinity and selective compounds will be tested for N/OFQ-like or N/OFQ inhibitory activity in mice and rats in models of analgesia, modulation of opiate tolerance, and reinforcing effects of drugs of abuse, with the goal of developing these compounds into novel therapeutics as non-addicting, analgesics, anxiolytics, or drug abuse medications. These studies should provide an improved understanding of the structural requirements for ORL1-selective ligands and the similarities and differences between the ORL1 and opioid receptors. As a complement to our study of ORL function, we will also synthesize a known high affinity ORL ligand as a precursor for PET studies in collaboration with John Hopkins University.